Since it is not a heavily planted tank there is going to be major issues with a pH that low. You see, nitrification slows way down when pH gets into the lower sixes and basically stops all together at six and below. With that low of a pH there will be no bio-filter to process waste, the bacteria simply cannot live. There will need to be something to take its place, like heavy planting or daily (or more) large water changes.
I don't keep Discus but keeping that low of a pH sounds like a breeder's thing to me, and they do massive water changes more than once a day to keep the water healthy (or they have elaborate water changing systems in place).
